This invention relates to a system for controlling the rotation of a spindle, and more particularly to a spindle rotation control system which is adapted to rotate the spindle of a machine tool at a commanded speed, to stop the spindle at a commanded position with a high accuracy, and to increase the rigidity at which the spindle is held when at rest.
Some machine tools which are known in the art have an automatic tool changing function that allows machining to be performed automatically while a variety of tools are changed automatically. The tool changing operation proceeds as follows. First, a magazine holding a number of tools is revolved to bring a vacant tool holding portion of the magazine into position directly above a spindle mechanism. The spindle mechanism, which is grasping an old tool to be exchanged for a new one, is then projected forwardly, after which the magazine positioned above the spindle mechanism is lowered to permit the old tool to be received and grasped by the vacant tool holding portion of the magazine. The spindle mechanism is then retracted so that the old tool separates from the spindle, thus transferring the old tool to the magazine. Next, the magazine is reolved to bring a desired new tool into position in front of the spindle, and the spindle mechanism is projected forwardly to receive and grasp the new tool. Finally the magazine is raised away from the spindle to complete the tool change operation.
It is required in the tool change mechanism of the foregoing type that the fitting portions of the spindle and a tool be mated accurately during the changing of tools. In other words, if a specified point on the spindle is not stopped accurately at a predetermined rotational position, the tool changing operation cannot proceed smoothly. To this end, machine tools having the conventional automatic tool change function are provided with a photoelectric detector or with a limit switch mechanism for detecting the position of a key on the spindle. The arrangement is such that the spindle is brought to a stop at the predetermined rotational position by the application of a mechanical brake which is actuated in response to a signal from the detecting means. Then, with the spindle stopped at the predetermined position, a pin projecting from the spindle is engaged with a keyway to fix and position the spindle accurately. Since this method makes use of the mechanical pin mechanism, however, the operator may accidentally apply an excessive force to the pin and cause it to bend. In such a case it would be impossible to stop the spindle at the predetermined rotational position, thereby giving rise to an occasion where the changing of tools could not be performed smoothly. Avoiding this situation usually entails troublesome maintenance and inspection work as well as the frequent replacement of the pin.
The present inventors have already proposed a spindle rotation control system which enables a spindle to be stopped at a predetermined rotational position with a high accuracy without the use of a mechanical brake or mechanical stopping mechanism, and which permits the spindle to be rotated at a commanded speed. The previously proposed system is illustrated in FIGS. 1 through 4. FIG. 1 shows a block diagram of a servo system employed in controlling spindle rotation, FIG. 2 is an illustrative view useful in explaining the operation of the servo system, FIG. 3 is a block diagram of a circuit for generating a position deviation signal, and FIG. 4 is a waveform diagram of signals associated with the circuit of FIG. 3.
Referring to FIGS. 1 and 2, there is provided a speed control circuit 1, a DC motor 2, a tachometer generator 3 for generating a voltage in accordance with the speed of the DC motor 2, and an orientation control circuit 4 for producing a voltage in accordance with a deviation between a commanded stopping position and the actual position of a spindle. Numeral 5 denotes a tool, 6 a spindle mechanism on which the tool is mounted, and 7 a spindle which is coupled to the DC motor 2 via a belt 8 (or gears). A rotational position detector 9, such as a resolver or a position coder which is adapted to generate a pulse whenever the spindle rotates by a predetermined angle, is connected directly to the spindle 7 for producing a signal in accordance with the rotational position of the spindle. In the arrangement described hereinafter, the rotational position detector adopted will be the position coder. Numeral 10 denotes a changeover switch. In FIG. 2, an orientation portion 11 must be located at a predetermined rotational position in order for a tool to be changed smoothly.
The movable contact of the changeover switch 10 (FIG. 1) is connected to the side a when the tool 5 performs a machining operation, so that the speed control circuit 1 receives a command speed signal CV, from a command speed signal generating circuit (not shown). The speed control circuit 1 also receives from the tachometer generator 3 an analog actual speed signal AV of a voltage level which is in accordance with the actual speed of the DC motor 2 as measured by the tachometer. The speed control circuit 1 is operable to produce an analog voltage in accordance with the deviation between the command speed signal CV and the actual speed signal AV, and to apply this analog voltage to the DC motor 2 to regulate its speed to the command speed. Thus, the speed control circuit 1, DC motor 2, tachometer generator 3 and a feedback line FL form a speed control feedback loop which functions to regulate the DC motor as described. This arrangement is well known in the art and need not be described in more detail.
When the machining work is completed and the DC motor 2 is to be stopped, the command speed signal CV is switched over to a value such as zero volts, and the speed of the motor is reduced while applying an electrical brake thereto. Then, immediately before the motor comes to rest, namely at such time that the speed of the motor has reached a fairly low level, an orientation command signal CPC is applied to the changeover switch 10, so that the movable contact of the switch is changed over from the side a to the side b.
The orientation control circuit 4 is adapted to produce a position deviation signal RPD, which is an analog voltage, in accordance with the deviation between a commanded stopping position which has been predetermined, and the actual rotational position of the spindle.
Reference will be had to FIGS. 3 and 4 to describe the operation of the orientation control circuit 4 for a case in which there is but one stopping position for the orientation portion 11 on the spindle 7. The arrangement is such that the position coder 9 produces a single pulse RP for each revolution of the spindle, and pulses PP each one of which is produced whenever the spindle rotates by a predetermined angle, the position coder 9 generating a total of N pulses for each single revolution of the spindle 7. The position coder 9 is mounted on the spindle 7 in such a manner that it issues the one-revolution pulse RP at such time that the orientation portion 11 on the spindle has rotated 180.degree. from the commanded stopping position STP, shown in FIG. 2. A counter 41 shown in FIG. 3 is set to the numerical value N upon the generation of the pulse RP, and then has this preset value decremented by each pulse PP that subsequently arrives from the position order. A digital-to-analog converter (referred to as a DA converter hereinafter) 42 converts the output of the counter 41 into an analog voltage DAV which is applied to an analog subtractor 43, the latter producing a difference voltage SV between the analog voltage DAV and a constant voltage Vc. Accordingly, if the voltage Vc is set to 1/2 the peak value of the analog voltage DAV from the DA converter, the difference voltage SV will have a sawtooth waveform that crosses the zero level at such time that 180.degree. is covered by the spindle from the generation of the pulse RP, as shown in FIG. 4. Since the commanded stopping position of the spindle is displaced by exactly 180.degree. from the point at which the pulse RP is generated, as described above, the orientation portion 11 on the spindle reaches the commanded stopping position at the moment the difference voltage SV crosses the zero level. It should be noted that the difference voltage SV is proportional to the position deviation signal RPD (FIG. 1).
Therefore when the changeover switch 10 in FIG. 1 is changed over to the b side, the speed control circuit 1 delivers a difference voltage between the position deviation signal RPD and the actual speed signal AV, whereby positional servo control is executed to make the position deviation signal RPD zero. Thus, the speed control circuit 1, DC motor 2, spindle 7, position coder 9, orientation control circuit 4 and changeover switch 10 form a position control feedback loop. If the orientation portion 11 on the spindle 7 is oriented as shown in FIG. 2(a), the spindle 7 will rotate counterclockwise and the orientation portion 11 will stop correctly at the commanded stopping position STP. Similarly, if the orientation portion 11 is oriented as shown in FIG. 2(b), the spindle will rotate clockwise and the orientation portion will stop correctly at the commanded stopping position.
Thus the previously proposed system rotates the spindle correctly at the commanded speed during rotation, and stops the spindle at the commanded stopping position when the spindle is to be stopped.
The DC motor employed in the above system to drive the spindle has a large inertia, so that it is necessary to reduce the gain of the speed control loop in view of system stability, that is, in order to preclude spindle overshoot and hunting. In other words, since this inertia is 5 to 20 times that of a DC motor employed in a feed servo system in which steady-state deviation and follow-up deviation pose problems, the gain of the spindle speed control loop is considerably low as compared to the gain of the feed servo system. This means that the spindle remains at rest with little rigidity and is likely to be turned by an externally applied force, such as may result from contact with the operator, or is likely to be rotated along with the motor if the latter is subjected to a mechanically eccentric load. This alters the rotational position at which the spindle is stopped and prevents tools from being changed smoothly. Furthermore, if the spindle orientation control circuit is applied to an apparatus such as a turning center that has a spindle indexing function, the spindle is likely to move during a cutting operation owing to the low rigidity of the spindle. This makes it impossible to machine a workpiece accurately. It is conventional practice, therefore, to make use of mechanical means such as a pin to prevent spindle rotation, but this complicates both the operating procedure and the mechanism itself.